The present invention relates to an active electro-optical device for detecting obstacles, in particular for use in autonomous navigation systems for ground moving or flying units with multiple degrees of freedom.
In particular, the invention relates to a device for detecting obstacles based on the measurement of the optical flow.
The effectiveness of an autonomous navigation system depends on its ability to reach a determined position and/or orientation in space. One of the main problems for a device of this kind consists of perceiving the surrounding environment and reacting in timely fashion.
Electro-optical devices for detecting obstacles based on the measurement of the optical flow have already been proposed. The optical flow indicates the speed whereby elements with different contrast move in a scene, as a result of the relative motion between an observer and the objects present in the scene. If one considers an object moving with a relative speed v with respect to an observer and at an orthogonal distance d with respect to the observer, the optical flow is given by the ratio between the speed v and the distance d.
Therefore, the optical flow measurement depends on the following factors:                resolution, field of view, frame-rate and sensitivity of the receiving means,        distance between the receiver means and the objects present in the scene,        relative speed between the receiver means and the objects present in the scene.        
Electro-optical devices for detecting obstacles based on the measurement of the optical flow are described for example in the following patent documents: U.S. Pat. No. 5,717,792, U.S. Pat. No. 5,257,209, EP 631109, U.S. Pat. No. 5,798,796 and EP 436213.
Algorithms for measuring the optical flow are typically implemented in passive electro-optical devices, i.e. devices which detect the radiation emitted by sources not included in the system (for instance, sun, moon, artificial lighting system, etc.) and reflected by the scene. To simplify computations, typically a small number of distinct sensors are used, whereon the optical flow is computed. The use of receiver means constituted by a great number of sensitive elements (for instance CCD or CMOS vision sensors) would require powerful computational units to implement the artificial vision algorithms. It must be kept in mind that the scene viewed by a passive vision sensor is usually very complex, since it includes objects positioned at a distance which may vary from a few centimetres to infinity. The scene therefore generates an enormous quantity of information about the optical flow of the objects present. The video information of the scene acquired by the optical sensor must then be filtered in the space and frequency domain to reduce the quantity and complexity of the data. Only after the information has been conveniently processed can it be used for optical flow computation.